A computer hard disk drive comprises a magnetic memory disk mounted on a spindle which is driven by a motor to rotate the magnetic disk at high speed. A read/write head, kept in close proximity to the surface of the rotating magnetic disk, reads or writes data on the magnetic disk. The read/write head is separated from the surface of the magnetic disk by an air bearing created by the high speed rotation of the magnetic disk. The read/write head flies on this air bearing, e.g., at a height of approximately one microinch. The closer the read/write head is to the surface of the magnetic disk, the more information may be written on the disk. Thus, it is desirable for the read/write head to fly as close as possible to the surface of the magnetic disk.
Typical memory disks comprise a substrate that is plated with a hard material such as a nickel phosphorus alloy. The nickel phosphorus is then textured or roughened. An underlayer, a magnetic alloy or magnetic-optical material, and a protective overcoat are then deposited on the nickel phosphorus, e.g., by sputtering. As mentioned above, the disk manufacturing process leaves the surface of the disk in a slightly roughened condition. The precision with which the read/write head flies over the magnetic disk requires that care is taken during manufacturing to assure that there are no protrusions or asperities on the disk surface that may interfere with the read/write head. A protrusion on the surface of the disk that contacts the read/write head during use may damage the head or the disk.
Accordingly, during manufacturing of magnetic or magnetic-optical disks, tests are performed with "glide heads" to determine if there are any asperities, voids or contamination that might interfere with the read/write head. Accurate testing of disks for such defects assures that the disk manufacturer does not unnecessarily reject good quality disks or pass on poor quality disks that may later fail.
During testing, the glide head must fly over the surface of the disk at a height no greater than the minimum fly height of the read/write head. FIG. 1 illustrates a glide head 1 flying over the surface of a magnetic disk 2. Disk 2 spins in the direction of arrow 3 about a spindle 4. Glide head 1 is connected to a suspension arm 5, which maintains the position of glide head 1 relative to disk 2. Suspension arm 5 is controlled by an actuator 7, such as a stepper-motor actuator or a voice-coil actuator, which moves glide head 1 laterally over the surface of magnetic disk 2 in the direction of arrow 6. The lateral movement of glide head 1 is slow relative to the high speed rotation of magnetic disk 2. Similar to a read/write head, glide head 1 flies over an air bearing that is created by the high speed rotation of magnetic disk 2.
FIG. 2 is a side view of magnetic disk 2 with a down facing glide head 1A and an up facing glide head 1B flying over and testing surfaces 2A and 2B of magnetic disk 2, respectively. Air bearings 8A, 8B, created by the high speed rotation of magnetic disk 2, lie between glide heads 1A and 1B and surfaces 2A and 2B, respectively. As in FIG. 1, glide heads 1A and 1B are connected to suspension arms 5A, 5B. Arms 5A, 5B are controlled by actuator 7 to laterally move glide heads 1A, 1B over surfaces 2A, 2B of magnetic disk 2 in the direction of arrow 6.
FIG. 3 shows glide head 1 flying over a section of magnetic disk 2 rotating in the direction of arrow 3. The roughened texture of top surface 2A of magnetic disk 2 is schematically shown in FIG. 3. FIG. 4A shows a bottom surface of down facing glide head 1A. FIG. 4B shows a trailing side 15 of down facing glide head 1A. As shown in FIGS. 4A and 4B, glide head 1A comprises a slider 9, a suspension arm 5 connected to a top surface of slider 9, and a transducer 10, such as a piezoelectric crystal. (Transducer 10 and suspension arm 5 are schematically represented in FIG. 4B.) Slider 9 comprises an inside rail 11, an outside rail 12, and a wing 13. Inside rail 11 and outside rail 12 both have forward tapered ends 14, which are tapered at an angle less than one degree from horizontal (typically an angle between thirty minutes and fifty minutes). Tapered ends 14 provide lift to glide head 1A. Wing 13 provides additional surface area to the top surface of slider 9 upon which transducer 10 is mounted. When rail 11 or 12 impact an asperity or contamination on disk 2 or sink in response to encountering a void, transducer 10 converts the mechanical energy from the event into an electrical signal which can be measured. Generally, however, inside rail 11 generates a stronger signal output voltage when detecting a defect than outside rail 12 when at the same fly height. Accordingly, it is difficult to determine when outside rail 12 is detecting a defect. Also, when a signal is generated by transducer 10, it is difficult to know the size of the defect that caused the signal, because one cannot know whether the signal was created by an encounter with the more sensitive inside rail 11 or the less sensitive outside rail 12.
Because tapered ends 14 of inside rail 11 and outside rail 12 create lift, it is important that as slider 9 moves laterally across the rotating surface of magnetic disk 2, both inside rail 11 and outside rail 12 remain over the surface of magnetic disk 2. In other words, one cannot move slider 9 such that outside rail 12 extends past the outer circumference of disk 2. If outside rail 12 is moved beyond the outer circumference of disk 2, slider 9 will lose its lift under outside rail 12 and will roll, causing slider 9 to contact magnetic disk 2. Accordingly, only outside rail 12 can detect asperities over the outermost portion of the surface of magnetic disk 2. Obviously, the surface of disk 2 adjacent the outer circumference must be tested for asperities. Thus, a glide head that can accurately test the outermost portion of the surface of a magnetic disk without losing its lift is needed.
The distance that slider 9 may move laterally outward along the surface of magnetic disk 2 is determined by the width of rails 11 and 12. In order to cover the entire surface area on magnetic disk 2, slider 9 is moved laterally, step by step, across the surface of magnetic disk 2. Each step must be at least slightly less than the width of one rail in order to test the entire surface of the disk for defects. Accordingly, in order to minimize the time necessary to test each magnetic disk, a glide head with wide rails is desirable.